In electronics systems it is desirable to have the source (system based pulser and receiver) and load (transducer) impedances be controlled.
1) To optimize power transfer to a transducer it is necessary to have the transducer match the complex conjugate impedance characteristics of the system.
2) To minimize reflections in the transmission line between the system and transducer, it is necessary to match the transducer impedance to the transmission line impedance.
3) To minimize output power variation, it is desirable to have a consistent transducer and system so that power transfer variation is minimized.
Ideally, for an ultrasound system, the source impedance has a controlled characteristic impedance with a phase angle of zero degrees. A cable that matches that impedance is used to connect the source to the transducer. Typical values for the impedances are 50-100 ohms. However, a piezoelectric element by itself will typically have an impedance that is very different from this, and that varies significantly from unit to unit.